Previous composite insulating materials intended for use in such applications as on orbital reentry vehicles, such as the Space Shuttle, have consisted of a coating in combination with a low density insulation substrate. Examples of such composites are disclosed in the following references: Beasley, R. M., Izu et al., "Fabrication and Improvement of LMSC's All-Silica RSI", Tech. Report No. NASA TMX-2719, November 1972; Musikant, S., Magin III, F. P., and Gebhart, J. J., "Development of REI Mullite (Reusable External Insulation) for Application to the Space Shuttle", National SAMPE Technical Conference on Space Shuttle Materials, Vol. 3 SAMPE, pp. 413-445 (1971); Tanzilli, E. A., Musikant, S. M., Bolinger, P. N., and Brazel, J. P., "Optimization of REI-Mullite Physical Properties", NASA TMX-2719, pp. 227-260 (1972); Gebhart, J. J. and Gorsuch, P. D., "Processing of Rigidized REI-Mullite Insulative Composites", Tech. Rept. No. NASA TMX-2719, pp. 17-61 (1973); Rusert, E. L., Christensen, H., "HCF-External Thermal Insulation for Space Shuttle Thermal Protection System", National SAMPE Technical Conference on Space Shuttle Materials, Vol. 3. SAMPE, pp. 403-413 (1971); Plank, P. P., Feldman, A. et al., "MAR-SI, Martin Surface Insulation", NASA TM X-2719, Vol. 1, 1972. A borosilicate glass, Reaction Cured Glass (RCG), was chosen as the coating for the silica type of Reusable Surface Insulation (RSI) selected as the heat shield for the Space Shuttle, as disclosed in Goldstein, H. E. et al., U.S. Pat. No. 4,093,771, issued June 6, 1978 and Goldstein et al., "Reaction Cured Borosilicate Glass Coating for Low Density Fibrous Silica Insulation" in Borate Glasses: Structure, Properties, Applications, Plenum Publishing Corp., 623-634 (1978). This coating was prepared by blending an emittance agent, silicon tetraboride, with a specially prepared borosilicate glass powder, composed of 94% by weight silica and 6% by weight boron oxide, and an ethanol carrier in a ball mill. The resulting slurry was sprayed on the silica substrate. The application characteristics of the mixed slurry were optimized to limit sagging during spraying and limit penetration of the slurry into the low density insulation. After drying, the coating was sintered (fused) to a "theoretical" density of 2.2 g/cc at 1220.degree. C. (2225.degree. F.). The "as fired" RCG coating weighs 0.07 g/sq.cm. (0.15 lbs/sq.ft.) and is approximately 0.3mm (0.013 in) thick and has been applied to advanced insulation systems. Further details on these composites and their use are provided in the following references: Leiser et al., U.S. Pat. No. 4,148,962, issued Apr. 10, 1979; Leiser, D. B., Smith M., and Goldstein, H. E., "Development in Fibrous Refractory Composite Insulation", American Ceramic Society Bulletin, 60, No. 11, pp. 1201-1204 (1981); Leiser, D. B., Smith M., and Stewart,D. A., "Options for Improving Rigidized Ceramic Heatshields", Ceramic Engineering and Science Proceedings, 6, No. 7-8, pp. 757-768 (1985) and Leiser, D. B., Smith M., and Stewart,D. A., "Effect of Fiber Size and Composition on Mechanical and Thermal Properties of Low Density Ceramic Composite Insulation Materials", NASA CP 2357, pp. 231-244 (1984). Other references relating to composite insulating materials include Fletcher et al., U.S. Pat. No. 3,953,646, issued Apr. 27, 1976; Fletcher et al., U.S. Pat. No. 3,955,034, issued May 4, 1976 and Johnson et al., U.S. Pat. No. 4,612,240, issued Sept. 16, 1986.